1. Field of the Invention
The invention relates to autopilots for marine vessels, particularly with respect to the heading keeping function thereof.
2. Description of the Prior Art
Numerous autopilots for marine vessels are known in the prior art that include a weather adjustment function. U.S. Pat. No. 3,604,907, issued Sept. 14, 1971 entitled "Steering Command Computer for Navigable Craft", discloses a dual gain autopilot with a semi-automatic weather adjustment function. A manually operated weather adjustment control was set by the operator in accordance with the nominal peak yaw motion experienced by the vessel in response to the severity of the seas. If the yaw motion of the ship exceeded the manual setting, the autopilot gain was increased to restore the ship to the ordered heading. Below the setting a low autopilot gain was utilized for heading keeping. In practice, such manual controls were often set incorrectly or were not set at all. In the prior art autopilots, such manual controls were adjusted to endeavor to optimize performance for varying wind and sea conditions as well as changing ship speeds. With such manual adjustments, however, the operator is frequently unable to realize optimum conditions since the time constants on large vessels are fairly long. With such vessels, it may be necessary to steam for periods of approximately ten minutes before the operator can determine the effect of a manual adjustment. The adjustments, therefore, become a matter of trial and error with a consequent loss in operating efficiency and increase in fuel consumption.
A prior art autopilot endeavoring to provide improved performance by automatically adjusting the autopilot gain in accordance with various sea states and encounter frequencies while maintaining critical damping of the heading control loop is described in U.S. Pat. No. 4,074,648, issued Feb. 21, 1978 entitled "Adaptive Autopilot for Marine Vessels". In the autopilot of said U.S. Pat. No. 4,074,648, the sensitivity of the steering system of the vessel is adjusted to accommodate changes in speed, sea and wind conditions to provide a balance between yaw motion and rudder activity. The autopilot computes RMS yaw motion and RMS rudder motion and adjusts the autopilot gain to minimize the drag function set forth in the patent. This approach requires detailed knowledge of the dynamic coefficients of the vessel and the dynamic performance characteristics thereof. Extensive computer simulation and analysis of ship steering characteristics is required to formulate the algorithm utilized to optimize the operating efficiency of the vessel and is therefore impractical for most ships. Furthermore, if the simulations do not match the actual performance of the vessel, there are no corrections or calibrations available to adjust the predicted performance to the actual performance so that the operating efficiency of the vessel may be compromised.
Another approach to solving the problem of automatically adjusting the autopilot to accommodate various sea states and encounter frequencies is described in pending U.S. patent application Ser. No. 692,465, now U.S. Pat. No. 4,692,868, filed Jan. 18, 1985 entitled "Adaptive Autopilot". Although the autopilot of said Ser. No. 692,465 operated successfully most ships, the device required sailing at constant headings for long periods of time with few heading changes. The autopilot utilized a sequence of trial and error iterative gain changes, RMS yaw motion measurements and comparisons with previous results until the gain was reduced to the level whereby RMS yaw motion increased significantly. Measurement periods were necessarily long, five minutes or longer, because yaw periods tended to be long. Several measurement periods were required to determine the least useable gain and it was necessary to restart the process when a heading change was effected. The autopilot of said Ser. No. 692,465 required significant operating time at non-optimum gain because of the lengthy search procedures utilized.